Calculation of biological effectiveness of SOBP proton beams: a TOPAS Monte Carlo study.

Objective.This study aims to investigate the biological effectiveness of Spread-Out Bragg-Peak (SOBP) proton beams with initial kinetic energies 50-250 MeV at different depths in water using TOPAS Monte Carlo code.Approach.The study modelled SOBP proton beams using TOPAS time feature. Various LET-based models and Repair-Misrepair-Fixation model were employed to calculate Relative Biological Effectiveness (RBE) for V79 cell lines at different on-axis depths based on TOPAS. Microdosimetric Kinetic Model and biological weighting function-based models, which utilize microdosimetric distributions, were also used to estimate the RBE. A phase-space-based method was adopted for calculating microdosimetric distributions.Main results.The trend of variation of RBE with depth is similar in all the RBE models, but the absolute RBE values vary based on the calculation models. RBE sharply increases at the distal edge of SOBP proton beams. In the entrance region of all the proton beams, RBE values at 4 Gy i.e. RBE(4 Gy) resulting from different models are in the range of 1.04-1.07, comparable to clinically used generic RBE of 1.1. Moving from the proximal to distal end of the SOBP, RBE(4 Gy) is in the range of 1.15-1.33, 1.13-1.21, 1.11-1.17, 1.13-1.18 and 1.17-1.21, respectively for 50, 100, 150, 200 and 250 MeV SOBP beams, whereas at the distal dose fall-off region, these values are 1.68, 1.53, 1.44, 1.42 and 1.40, respectively.Significance.The study emphasises application of depth-, dose- and energy- dependent RBE values in clinical application of proton beams.

Microdosimetry-based investigation of biological effectiveness of252Cf brachytherapy source: TOPAS Monte Carlo study.

Objective.To investigate biological effectiveness of252Cf brachytherapy source using Monte Carlo-calculated microdosimetric distributions.Approach.252Cf source capsule was placed at the center of the spherical water phantom and phase-space data were scored as a function of radial distance in water (R= 1-5 cm) using TOPAS Monte Carlo code. The phase-space data were used to calculate microdosimetric distributions at 1µm site size. Using these distributions, Relative Biological Effectiveness (RBE), mean quality factor (Q̅) and Oxygen Enhancement Ratio (OER) were calculated as a function ofR.Main results.The overall shapes of the microdosimetric distributions are comparable at all the radial distances in water. However, slight variation in the bin-wise yield is observed withR. RBE,Q̅and OER are insensitive to R over the range 1-5 cm. Microdosimetric kinetic model based RBE values are about 2.3 and 2.8 for HSG tumour cells and V79 cells, respectively, whereas biological weighting function-based RBE is about 2.8. ICRP 60 and ICRU 40 recommendation-basedQ̅values are about 14.5 and 16, respectively. Theory of dual radiation action based RBE is 11.4. The calculated value of OER is 1.6.Significance.This study demonstrates the relative insensitivity of RBE,Q̅and OER radially away from the252Cf source along the distances of 1-5 cm in water.

A machine learning approach for correcting glow curve anomalies in CaSO4:Dy-based TLD dosimeters used in personnel monitoring.

The study presents a novel approach to analysing the thermoluminescence (TL) glow curves (GCs) of CaSO4:Dy-based personnel monitoring dosimeters using machine learning (ML). This study demonstrates the qualitative and quantitative impact of different types of anomalies on the TL signal and trains ML algorithms to estimate correction factors (CFs) to account for these anomalies. The results show a good degree of agreement between the predicted and actual CFs, with a coefficient of determination greater than 0.95, a root mean square error less than 0.025, and a mean absolute error less than 0.015. The use of ML algorithms leads to a significant two-fold reduction in the coefficient of variation of TL counts from anomalous GCs. This study proposes a promising approach to address anomalies caused by dosimeter, reader, and handling-related factors. Furthermore, it accounts for non-radiation-induced TL at low dose levels towards improving the dosimetric accuracy in personnel monitoring.

Monte Carlo Study on Dose Distributions Around 192Ir, 169Yb, and 125I Brachytherapy Sources Using EGSnrc-based egs_brachy User-code.

Introduction: As per the recommendations of the American Association of Physicists in Medicine Task Group 43, Monte Carlo (MC) investigators should reproduce previously published dose distributions whenever new features of the code are explored. The purpose of the present study is to benchmark the TG-43 dosimetric parameters calculated using the new MC user-code egs_brachy of EGSnrc code system for three different radionuclides 192Ir, 169Yb, and 125I which represent high-, intermediate-, and low-energy sources, respectively. Materials and Methods: Brachytherapy sources investigated in this study are high-dose rate (HDR) 192Ir VariSource (Model VS2000), 169Yb HDR (Model 4140), and 125I -low-dose-rate (LDR) (Model OcuProsta). The TG-43 dosimetric parameters such as air-kerma strength, S k, dose rate constant, Λ, radial dose function, g(r) and anisotropy function, F(r,θ) and two-dimensional (2D) absorbed dose rate data (along-away table) are calculated in a cylindrical water phantom of mass density 0.998 g/cm3 using the MC code egs_brachy. Dimensions of phantom considered for 192Ir VS2000 and 169Yb sources are 80 cm diameter ×80 cm height, whereas for 125I OcuProsta source, 30 cm diameter ×30 cm height cylindrical water phantom is considered for MC calculations. Results: The dosimetric parameters calculated using egs_brachy are compared against the values published in the literature. The calculated values of dose rate constants from this study agree with the published values within statistical uncertainties for all investigated sources. Good agreement is found between the egs_brachy calculated radial dose functions, g(r), anisotropy functions, and 2D dose rate data with the published values (within 2%) for the same phantom dimensions. For 192Ir VS2000 source, difference of about 28% is observed in g(r) value at 18 cm from the source which is due to differences in the phantom dimensions. Conclusion: The study validates TG-43 dose parameters calculated using egs_brachy for 192Ir, 169Yb, and 125I brachytherapy sources with the values published in the literature.

Microdosimetry-based relative biological effectiveness calculations for radiotherapeutic electron beams: a FLUKA-based study.

Based on FLUKA, the present study is aimed at calculating the microdosimetric distributions of electron beams (6, 12 and 18 MeV) for radiotherapy as a function of depth in water at a site-size of 1 µm using a tissue-equivalent proportional counter (TEPC). Using the calculated microdosimetric distributions, the depth-specific relative biological effectiveness (RBE) of electron beams used in radiotherapy is calculated based on the theory of dual radiation action (TDRA) and the microdosimetric kinetic model (MKM). The TDRA-based calculation shows the variation of RBE of an electron beam with the absorbed dose and depth in water. In this study, we compared the RBE values calculated based on the TDRA and MKM. The FLUKA-based microdosimetric distributions in water obtained using the pre-calculated electron fluence spectra resulted in an improvement in the computational efficiency by a factor of 110 when compared with a full simulation. Depending on the beam energy and depth of water, RBETDRA was in the range 0.67-0.78. RBEMKM at 10% survival of HSG tumor cells was 0.84, which was nearly independent of the depth and beam energy.

Monte Carlo-based dosimetric studies of a locally developed170Tm LDR brachytherapy seed source.

170Tm is being explored as a source for applications in brachytherapy. Although it has adequate physical properties, such as a short half-life (128.6 d), high specific activity and a mean photon energy of about 66 keV, it has a drawback of low photon yield (only about six photon emissions/100 beta emissions). The objective of this work is to study the dosimetric characteristics of a locally developed170Tm brachytherapy seed source using the Monte Carlo-based EGSnrc code system. In this study, we calculate the dose rate constant, air-kerma strength, radial dose function, anisotropic function and 2D dose-rate distributions in water. Separate simulations are carried out by considering the photon (gamma and characteristic x-ray) and beta spectra of the source. For regions close to the source (surface of the source

TISSUE-EQUIVALENCE OF H2 GAS FOR MICRODOSIMETRY IN NEUTRON FIELDS: A GEANT4 MONTE CARLO STUDY.

Hydrogen (H2) as filling gas in Tissue-equivalent proportional counter (TEPC) for measurements of microdosimetric distributions in neutron fields is investigated using the Monte Carlo-based Geant4 toolkit. The neutron fields considered are monoenergetic neutrons (1 keV-14 MeV) and ISO reference neutron sources 241Am-Be, 241Am-B, 252Cf and 252Cf + D2O. Based on the energy deposited in the gas cavities (tissue-equivalent (TE) propane and H2) of the TEPC, density scaling correction, ${f}_{\rho }$, to be applied on H2 gas to achieve tissue-equivalence is calculated for the above sources at 1µm site size. The calculated value of ${f}_{\rho }$ for the ISO-neutron sources is 0.40, which suggests that energy deposition in the gas cavities is predominantly due to crossers. In the case of monoenergetic neutrons, depending upon the energy, ${f}_{\rho }$ is in the range of 0.11-0.45. The study shows that the TE propane and H2-based microdosimetric distributions are comparable when the density of H2 is scaled appropriately. Mean quality factor $\overline{Q}$ calculated based on microdosimetric distribution increases initially with neutron energy up to 100 keV and thereafter decreases with energy. Depending upon the neutron energy, TE propane and H2-based $\overline{Q}$ values are comparable to within 3-15%, and for the ISO sources, the comparison is within 5-8%. For the ISO neutron sources, $\overline{Q}$ values are in the range of ~10-15.

ESTIMATION OF ACTUAL DOSE BASED ON BAYESIAN PROBABILISTIC APPROACH USING PERSONNEL MONITORING DOSE RECORDS.

In this study, the Bayesian probabilistic approach is applied for the estimation of the actual dose using personnel monitoring dose records of occupational workers. To implement the Bayesian approach, the probability distribution of the uncertainty in the reported dose as a function of the actual dose is derived. Using the uncertainty distribution function of reported dose and prior knowledge of dose levels generally observed in a monitoring period, the posterior probability distribution of the actual dose is estimated. The posterior distributions of each monitoring period in a year are convoluted to arrive at actual annual dose distribution. The estimated actual doses distributions show a significant deviation from reported annual doses particularly for low annual doses.

EGSnrc-based depth-dependent photon energy response and phantom scatter corrections for low-energy brachytherapy sources.

In the present study, beam quality correction, [Formula: see text], and phantom scatter correction, kphan(r), for low-energy brachytherapy sources, 131Cs, 125I, and 103Pd, are calculated using the Monte Carlo-based EGSnrc code system as a function of the distance along the transverse axis of the source. The solid-state detectors investigated are diamond, LiF, Li2B4O7, Al2O3, and radiochromic films, such as HS, EBT, EBT2, EBT3, RTQA, XRT, and XRQA. The solid phantoms investigated are polystyrene, PMMA, virtual water, solid water, plastic water (LR), A150, RW1, RW3, and WE210. For a given detector and brachytherapy source, [Formula: see text] is independent of distance in the water phantom. Meanwhile, for a given detector, kphan(r) depends on the distance from the source for the investigated solid phantoms. Moreover, the kphan(r) values do not change with the detector type for sources 131Cs, 125I, and 103Pd at all distances. The LR and A150 phantoms are water equivalent for the investigated distances of 1-5 cm. The phantoms including solid water, virtual water, and WE210 are not water-equivalent for distances beyond 1 cm. Furthermore, PMMA, polystyrene, RW1, and RW3 are not water equivalent.

MACHINE LEARNING ALGORITHMS FOR IDENTIFICATION OF ABNORMAL GLOW CURVES AND ASSOCIATED ABNORMALITY IN CaSO4:DY-BASED PERSONNEL MONITORING DOSIMETERS.

In the present study, machine learning (ML) methods for the identification of abnormal glow curves (GC) of CaSO4:Dy-based thermoluminescence dosimeters in individual monitoring are presented. The classifier algorithms, random forest (RF), artificial neural network (ANN) and support vector machine (SVM) are employed for identifying not only the abnormal glow curve but also the type of abnormality. For the first time, the simplest and computationally efficient algorithm based on RF is presented for GC classifications. About 4000 GCs are used for the training and validation of ML algorithms. The performance of all algorithms is compared by using various parameters. Results show a fairly good accuracy of 99.05% for the classification of GCs by RF algorithm. Whereas 96.7% and 96.1% accuracy is achieved using ANN and SVM, respectively. The RF-based classifier is recommended for GC classification as well as in assisting the fault determination of the TLD reader system.

APPLICABILITY OF PURE PROPANE GAS FOR MICRODOSIMETRY AT BRACHYTHERAPY ENERGIES: A FLUKA STUDY.

Applicability of pure propane gas for microdosimetric measurements at photon energies relevant in brachytherapy is studied using the Monte Carlo-based FLUKA code. Monoenergetic photons in the energy range of 20-1250 keV and brachytherapy sources such as 103Pd, 125I, 169Yb, 192Ir, 137Cs and 60Co are considered in the study. Using the calculated values of energy deposited in the sensitive region of LET-1/2 tissue-equivalent proportional counter filled with pure propane gas and tissue-equivalent propane gas, values of density scaling factor for the site sizes of 1 and 8 µm are obtained. The study shows that density of propane gas should be lowered by a factor of about 0.93 for 169Yb, 192Ir, 137Cs and 60Co sources for the site sizes of 1-8 µm. For 125I source, the density of propane gas requires a scaling of 0.93 for 1 µm site size, whereas for site sizes 2-8 µm, density need not be altered. 103Pd source does not require density scaling for site sizes 1-8 µm.

ESTIMATION OF UNCERTAINTY IN MEASUREMENT OF DOSE EQUIVALENT AT LABORATORY LEVEL USING CASO4 :Dy-BASED TLD BADGE SYSTEM IN INDIA.

The objective of this paper is to estimate the combined uncertainty in the measurement of dose equivalent at laboratory level using CaSO4:Dy-based thermoluminescent dosemeter badge system by including variations in the components of the system. The variability of performance of the system is analysed using random effects one way analysis of variance model. The model enables estimation of the overall variance of the performance of the sampled population. The population in the study comprises all possible indicated dose equivalents on irradiation of dosemeters to a specific dose equivalent and radiation quality. Coefficient of variation and combined uncertainty at 95% level of confidence in the measurement of Hp(10) due to S-Cs radiation quality are found to be 6.6 and 14.3%, respectively, at the dose level of 5.31 mSv. The above parameters in the measurement of in-use quantity, i.e. whole body dose or photon dose equivalent are found to be 7.4 and 16.4%, respectively. The performance of the monitoring system on relative response has been observed to be satisfactory. Various factors affecting the variability of performance of the system are identified for further improvement in coefficient of variation.

STUDY OF EFFECT OF CONSECUTIVE HEATING ON THERMOLUMINESCENCE GLOW CURVES OF MULTI-ELEMENT TL DOSEMETER IN HOT GAS-BASED READER SYSTEM.

The objective of this paper is to study the effect of consecutive heating of TL elements of a thermoluminescence dosemeter (TLD) card in hot N2 gas-based TLD badge reader. The effect is studied by theoretical simulations of clamped heating profiles of the discs and resulting TL glow curves. The simulated temperature profile accounts for heat transfer to disc from hot gas as well as radiative and convective heat exchanges between the disc and the surrounding. The glow curves are simulated using 10 component glow peak model for CaSO4:Dy using the simulated temperature profile. The shape of the simulated glow curves and trend in total TL signal of the three discs were observed to match closely with the experimental observations when elevated surrounding temperature was considered for simulation. It is concluded that the readout (heating) of adjacent TLD disc affects the surrounding temperature leading to the changes in temperature profile of the next disc.

MONTE CARLO-BASED INVESTIGATION OF MICRODOSIMETRIC DISTRIBUTION OF HIGH ENERGY BRACHYTHERAPY SOURCES.

FLUKA-based Monte Carlo calculations were carried out to study microdosimetric distributions in air and in water for encapsulated high energy brachytherapy sources (60Co, 137Cs, 192Ir and 169Yb) by simulating a Tissue Equivalent Proportional Counter (Model LET1/2) having sensitive diameter of 1. 27 cm for a site size of 1 µm. The study also included microdosimetric distributions of bare sources. When the sources are in air, for a given source, the source geometry does not affect the y¯F and y¯D values significantly. When the encapsulated 192Ir, 137Cs and 60Co sources are in water, y¯F and y¯D values increase with distance in water which is due to degradation in the energy of photons. Using the calculated values of y¯D, relative biological effectiveness (RBE) was obtained for the investigated sources. When 60Co, 137Cs and 192Ir sources are in water, RBE increases from 1.03 ± 0.01 to 1.17 ± 0.01, 1.24 ± 0.01 to 1.46 ± 0.02 and 1.50 ± 0.01 to 1.75 ± 0.03, respectively, when the distance was increased from 3-15 cm, whereas for 169Yb, RBE is about 2, independent of distance in water.

MEASUREMENT OF OPERATIONAL QUANTITIES Hp(0.07) AND Hp(3) FOR INDIGENOUSLY DEVELOPED 106Ru/106Rh SOURCE USING AN EXTRAPOLATION CHAMBER.

In the present study, a prototype 106Ru/106Rh source was fabricated using high level liquid waste from reactor fuel, fixed in a stainless steel housing with a window and backing made of silver. The study involves measurement of the operational quantities Hp(0.07), Hp(3) and the percentage depth dose (PDD) using an extrapolation chamber. It also involves determination of necessary correction factors to arrive at Hp(0.07) and Hp(3) following International Organisation for Standardisation (ISO) and methods suggested in literature. The study facilitates incorporation of the 106Ru/106Rh source as a beta reference source for quality assurance programme in TLD personnel monitoring as per the guidelines of ISO.

Dosimetry of indigenously developed 177Lu patch source for surface brachytherapy-Experimental and Monte Carlo methods.

This paper describes the evaluation of dosimetry characteristics of an in-house developed 177Lu skin patch source for treatment of non-melanoma skin cancer. A 177Lu skin patch source based on Nafion-115 membrane backbone containing 3.46 ± 0.01 mCi of activity was used. Activity measurement of the patch source was based on gamma ray spectrometry using a HPGe detector. The efficiencies of the HPGe detector were fitted using an orthogonal polynomial function. The absorbed dose rate to water at 5 µm depth in water was determined using an extrapolation chamber, EBT3 Gafchromic film and compared with Monte Carlo methods. The correction factors such as Bragg-Gray stopping power ratio of water-to-air and chamber wall material being different from water, needed to be applied on measurements for establishing the dose rate at 5 µm depth, were calculated using the Monte Carlo method. Absorbed dose rate at 5 µm depth in water (surface dose rate) measured using an extrapolation chamber and EBT3 Gafchromic film were 9.9 ± 0.7 and 8.2 ± 0.1 Gy h-1 mCi-1 respectively for the source activity of 3.46 ± 0.01 mCi. The surface dose rate calculated using the Monte Carlo method was 8.7 ± 0.2 Gy h-1 mCi-1, which agrees reasonably well with measurement. The measured dose rate per mCi offers scope for ascertaining treatment time required to deliver the dose for propitious therapeutic outcome. Additionally, on-axis depth dose and lateral dose profiles at 5 µm and 1 mm depth in water phantom were also calculated using the Monte Carlo method.

INVESTIGATION OF APPLICABILITY OF PURE PROPANE GAS FOR MICRODOSIMETRY AT NEUTRON FIELDS: A MONTE CARLO STUDY.

Applicability of pure propane gas for microdosimetric measurements in neutron fields was investigated using the FLUKA Monte Carlo code. Monoenergetic neutrons in the energy range 1 keV-20 MeV and the ISO-neutron sources such as 241Am-Be, 241Am-B, 252Cf and 252Cf + D2O were considered in the present study. The tissue-equivalent proportional counter (TEPC) simulated in the study was LET-1/2 (by Far West Technology) with site sizes 1, 2 and 8 µm. The study demonstrates that for a given site size, the TEPC filled with tissue-equivalent propane and pure propane gases produce similar microdosimetric distributions when the density of pure propane gas is lowered appropriately. For the ISO-neutron sources, the density of propane gas requires scaling by a factor 0.85. For the monoenergetic neutrons, depending upon the neutron energy, the values of scaling factors are in the range of 0.58-0.93.

Comparison of Measured and Monte Carlo Calculated Dose Distributions from Indigenously Developed 6 MV Flattening Filter Free Medical Linear Accelerator.

PURPOSE: Monte Carlo simulation was carried out for a 6 MV flattening filter-free (FFF) indigenously developed linear accelerator (linac) using the BEAMnrc user-code of the EGSnrc code system. The model was benchmarked against the measurements. A Gaussian distributed electron beam of kinetic energy 6.2 MeV with full-width half maximum of 1 mm was used in this study. METHODS: The simulation of indigenously developed linac unit has been carried out by using the Monte Carlo-based BEAMnrc user-code of the EGSnrc code system. Using the simulated model, depth and lateral dose profiles were studied using the DOSXYZnrc user-code. The calculated dose data were compared against the measurements using an RFA dosimertic system made by PTW, Germany (water tank MP3-M and 0.125 cm3 ion chamber). RESULTS: The BEAMDP code was used to analyze photon fluence spectra, mean energy distribution, and electron contamination fluence spectra. Percentage depth dose (PDD) and beam profiles (along both X and Y directions) were calculated for the field sizes 5 cm × 5 cm - 25 cm × 25 cm. The dose difference between the calculated and measured PDD and profile values were under 1%, except for the penumbra region where the maximum deviation was found to be around 3%. CONCLUSIONS: A Monte Carlo model of indigenous FFF linac (6 MV) has been developed and benchmarked against the measured data.

Monte Carlo Investigation of Photon Beam Characteristics and its Variation with Incident Electron Beam Parameters for Indigenous Medical Linear Accelerator.

PURPOSE: A Monte Carlo model of a 6 MV medical linear accelerator (linac) unit built indigenously was developed using the BEAMnrc user code of the EGSnrc code system. The model was benchmarked against the measurements. Monte Carlo simulations were carried out for different incident electron beam parameters in the study. MATERIALS AND METHODS: Simulation of indigenously developed linac unit has been carried out using the Monte Carlo based BEAMnrc user-code of the EGSnrc code system. Using the model, percentage depth dose (PDD), and lateral dose profiles were studied using the DOSXYZnrc user code. To identify appropriate electron parameters, three different distributions of electron beam intensity were investigated. For each case, the kinetic energy of the incident electron was varied from 6 to 6.5 MeV (0.1 MeV increment). The calculated dose data were compared against the measurements using the PTW, Germany make RFA dosimetric system (water tank MP3-M and 0.125 cm3 ion chamber). RESULTS: The best fit of incident electron beam parameter was found for the combination of beam energy of 6.2 MeV and circular Gaussian distributed source in X and Y with FWHM of 1.0 mm. PDD and beam profiles (along both X and Y directions) were calculated for the field sizes from 5 cm × 5 cm to 25 cm × 25 cm. The dose difference between the calculated and measured PDD and profile values were under 1%, except for the penumbra region where the maximum deviation was found to be around 2%. CONCLUSIONS: A Monte Carlo model of indigenous linac (6 MV) has been developed and benchmarked against the measured data.

FLUKA-BASED MONTE CARLO INVESTIGATION OF MICRODOSIMETRIC DISTRIBUTIONS OF TELECOBALT BEAM.

FLUKA-based Monte Carlo calculations of microdosimetric distributions in water phantom involving a walled spherical Tissue-Equivalent Proportional Counter filled with tissue-equivalent propane gas have been studied for an indigenously developed telecobalt machine. The simulated site size considered in the study was 2 µm. In the Monte Carlo calculations, field size was varied from 10 cm × 10 cm to 35 cm × 35 cm and the depth was varied as 5-20 cm. The study also includes calculation of microdosimetric distributions with a 30° wedge filter. The efficiency of the calculations was improved up to a factor of 26 by choosing appropriate cut off values for production and transport of electrons. The calculated microdosimetric distributions of telecobalt machine is distinctly different from that of a bare 60Co source which is attributed to the influence of scattered photons from the machine head and the water phantom.